Method for fabricating gate oxide film of semiconductor device

ABSTRACT

The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a gate oxide film of a semiconductor device by which semiconductor devices having different electrical characteristics can be implemented in the same chip. The present invention provides a method for fabricating a gate oxide film of a semiconductor device which includes the steps of: forming a screen oxide film on the top surface of a semiconductor substrate; forming an ion implantation mask on parts of the top surface of the screen oxide film; implanting nitrogen ions into the semiconductor substrate using the ion implantation mask; removing the ion implantation mask and the screen oxide film; forming an oxide film on the top surface of the semiconductor substrate; and annealing the semiconductor substrate in a N 2 O or O 3  atmosphere.

This application claims the benefit of Korean Application No.47183/1999, filed in the Republic of Korea on Aug. 16, 2000, which ishereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for fabricating asemiconductor device, and more particularly, to a method for fabricatinga gate oxide film of a semiconductor device by which semiconductordevices having different electrical characteristics can be implementedin the same chip.

2. Description of the Background Art

Recently, as the degree of integration of a semiconductor, inparticular, a DRAM (dynamic random access memory) increases, it is oftenthe case that a transistor of a memory cell unit and a transistor of aperipheral circuit have a different operating voltage with each other.In other words, the transistor of the memory cell unit fabricated with afine line width operates at a voltage less than 1.8V, and the transistorof the peripheral circuit operates at a voltage of 3.3V or 5V formatching with exterior system equipment.

Accordingly, as devices having different operating voltages are formedin the same chip, there occurs a problem that a gate oxide film of thetransistors formed in the same semiconductor chip must have differentthicknesses.

Methods conventionally known as a method for forming a gate electrodehaving different thicknesses in the same chip will now be described.

First, FIGS. 1A through 1D illustrate a method for fabricating a gateoxide film by a dual step oxidation process.

As illustrated in FIG. 1A, a semiconductor substrate 10 is prepared.

Next, as illustrated in FIG. 1B, a first gate oxide film 11 is formed onthe top surface of the semiconductor substrate 10.

Next, as illustrated in FIG. 1C, the first gate oxide film 11 of aportion on which a relatively thin gate oxide film is to be selectivelyetched and removed to thereby expose parts of the top surface of thesemiconductor substrate 10.

Next, as illustrated in FIG. 1D, a second gate oxide film 12 is formedon the top surface of the first gate oxide film 11 and the top surfaceof the semiconductor device 10.

Besides the above-said method using the dual step oxidation process,there is a method for fabricating a gate oxide film using an ionimplantation process. This method will now be described with referenceto FIGS. 2A through 2D and FIGS. 3A through 3D.

First, FIGS. 2A through 2D illustrates a method for fabricating a gateoxide film using a nitrogen ion implantation process.

As illustrated in FIG. 2A, a semiconductor substrate 20 is prepared.

Next, as illustrated in FIG. 2B, a screen oxide film 21 is formed on thetop surface of the semiconductor substrate 20. Then, an ion implantationmask 22 is formed on the screen oxide film 21 of a portion on which arelatively thick oxide film is to be formed. Then, nitrogen (N₂) ionsare implanted into the semiconductor substrate 20 of a portion being notcovered with the ion implantation mask 22.

Next, as illustrated in FIG. 2C, the screen oxide film 21 and the ionimplantation mask are removed.

Next, when a gate oxide film 23 is formed on the top surface of thesemiconductor substrate 20, as illustrated in FIG. 2D, a thin oxide film23 a is formed on a portion into which nitrogen ions are implanted,because oxidation is restrained, and a relatively thick oxide film 23 bis formed on a portion into which nitrogen ions are not implanted.

In addition, a method for fabricating a gate oxide film using a fluorideion implantation process will now be described with reference to FIGS.3A through 3D.

First, as illustrated in FIG. 3A, a semiconductor substrate 30 isprepared.

Next, as illustrated in FIG. 3B, a screen oxide film 31 is formed on thetop surface of the semiconductor substrate 30. Next, an ion implantationmask 32 is formed on the top surface of the screen oxide film 31 of aportion on which a relatively thin gate oxide film is to be formed.Then, fluoride ions are implanted into the semiconductor substrate 30using the ion implantation mask 32.

Next, as illustrated in FIG. 3C, the ion implantation mask 32 and thescreen oxide film 31 are removed.

Next as illustrated in FIG. 3D, a gate oxide film 33 having differentthicknesses is formed on the top surface of the semiconductor substrate30 by oxidation of the semiconductor substrate 30. That is, a thick gateoxide film is formed on the top surface of the semiconductor of aportion into which fluoride ions are implanted, and a thin gate oxidefilm is formed on a portion into which fluoride ions are not implanted.

However, the above-described conventional methods for fabricating a gateoxide film has the following problems. First, the method for fabricatinga gate oxide film by the dual step oxidation process has a complicatedprocedure, and a peripheral portion of a fabricated, thick gate oxidefilm becomes thinner and a breakdown is easily occurred on a thinnedportion.

Second, in case of the fluoride ion implantation process, since a largeamount of fluoride ions must be implanted in order to make the thicknessof a gate oxide film different according to its portion, thesemiconductor substrate is largely damaged to thus increase the amountof leakage current.

Third, the nitrogen implantation process is disadvantageous in that, incase of forming a gate oxide film on the top surface of thesemiconductor substrate into which nitrogen ions are implanted, the gateoxide film is degraded, although it is advantageous in that a smalleramount of ions can be implanted as to compared to the fluoride ionimplantation process, which rather decreases the amount of leakagecurrent.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method for fabricating agate oxide film of a semiconductor device having a small leakage currentamount and a high reliability by reducing the concentration of nitrogenin the gate oxide film when a gate oxide film having differentthicknesses according to its portion is fabricated on the top surface ofa semiconductor substrate using a nitrogen ion implantation process.

A method for fabricating a gate oxide film of a semiconductor deviceaccording to the present invention includes the steps of: forming ascreen oxide film on the top surface of a semiconductor substrate;forming an ion implantation mask on parts of the top surface of thescreen oxide film; implanting nitrogen ions into the semiconductorsubstrate using the ion implantation mask; removing the ion implantationmask and the screen oxide film; forming an oxide film on the top surfaceof the semiconductor substrate; and annealing the semiconductorsubstrate.

There is provided a method for fabricating a gate oxide film of asemiconductor device according to the present invention which furtherincludes a pre-annealing step after the ion implantation step.

A method for fabricating a gate oxide film of a semiconductor deviceaccording to the present invention includes the pre-annealing step by aprocess of annealing at 500-900° C. by a furnace annealing method.

In a further aspect of the invention, there is provided a method forfabricating a gate oxide film of a semiconductor device wherein thepre-annealing step is a process of annealing at 850-1200° C. by a rapidthermal annealing method.

In another aspect of the invention, there is provided a method forfabricating a gate oxide film of a semiconductor device wherein theoxide film formation step being a method for thermal oxidation at afurnace of 700-950° C.

A method for fabricating a gate oxide film of a semiconductor deviceaccording to the present invention includes the oxide film formationstep being a method for thermal oxidation at 850-1200° C. at the rapidthermal annealing method.

A method for fabricating a gate oxide film of a semiconductor deviceaccording to the present invention includes the annealing step beingperformed by means of the rapid thermal annealing method in a N₂Oatmosphere at a temperature of 900-1200° C. for about less than fiveminutes.

The annealing step can also be performed by means of the thermalannealing method in an O₃ atmosphere at a temperature of 400-1200° C.for about five minutes.

The annealing step can also be performed by means of the furnaceannealing method in a N₂O atmosphere at a temperature of 850-1200° C.for about one hour.

In another aspect of the invention, there is provided a method forfabricating a gate oxide film of a semiconductor device according to thepresent invention wherein the annealing step is performed by means ofthe rapid thermal annealing method in a N₂O atmosphere at a temperatureof 900-1200° C. for about less than five minutes.

Additional advantages, aspects and features of the invention will becomemore apparent from the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become better understood with reference tothe accompanying drawings which are given only by way of illustrationand thus are not limitative of the present invention, wherein:

FIGS. 1A through 1D are a process chart illustrating one example of amethod for fabricating a gate oxide film according to the conventionalart;

FIGS. 2A through 2D are a process chart illustrating another example ofa method for fabricating a gate oxide film according to the conventionalart;

FIGS. 3A through 3D are a process chart illustrating still anotherexample of a method for fabricating a gate oxide film according to theconventional art;

FIGS. 4A through 4D are a process chart illustrating a method forfabricating a gate oxide film according to the present invention;

FIG. 5 is a graph illustrating the results of the SIMS analysis afterannealing;

FIG. 6 is a graph illustrating the changes in leakage current afterannealing of a gate oxide film; and

FIG. 7 is a graph illustrating the characteristics of an oxide film,e.g., a graph illustrating the amount of electric charge accumulated inthe oxide film to the breakdown of the oxide film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will now be described withreference to the accompanying drawings.

First, as illustrated in FIGS. 4A and 4B, a screen oxide film 41 isformed on the top surface of a semiconductor device at a thickness ofless than 200 Å.

Next, as illustrated in FIG. 4B, an ion implantation mask 42 is formedon the top surface of the screen oxide film 41. The ion implantationmask 42 is formed only on the top surface of a portion on which arelatively thick gate oxide film is to be formed. Then, nitrogen ionsare implanted into the semiconductor substrate 40. At this time, the ionimplantation process is performed with the implantation amount ofnitrogen ions ranging from 5×10¹³ cm² to 5×10¹¹ cm² and the ionimplantation energy of 5-50 KeV.

Next, in order to prevent the damage to the semiconductor substrateoccurred due to the ion implantation process and move the distributionof nitrogen (N) atoms in the vicinity of the screen oxide film 41, apre-annealing process is performed. At this time, in case of furnaceannealing, the pre-annealing process is performed at a furnacetemperature of 500-900° C. for less than six hours. Meanwhile, in caseof rapid thermal annealing (RTA), the pre-annealing process is performedat 850-1200° C. for less than five minutes.

Next, as illustrated in FIG. 4C, the screen oxide film 41 and the ionimplantation mask 42 are removed.

Next, as illustrated in FIG. 4D, a gate oxide film 43 is formed on thetop surface of the semiconductor substrate 40. At this time, arelatively thin gate oxide film 43 a is formed on the top surface of thesemiconductor substrate of a portion into which nitrogen ions areimplanted, and a relatively thick gate oxide film 43 b is formed on thetop surface of the semiconductor substrate of a portion into whichnitrogen ions are not implanted. In addition, the oxide film formationprocess is a process of wet oxidation at a furnace temperature of700-950° C. by the rapid thermal annealing method, or a process of dryoxidation at a furnace temperature of 850-1200° C. by the same method.

Next, as illustrated in FIG. 4D, the semiconductor substrate 40 on whichthe gate oxide film 43 is fabricated is annealed. The conditions of theannealing process are as follows. In case of the rapid thermal annealingprocess, it is performed in a N₂O gas atmosphere at 850-1200° C. forabout less than five minutes. In case of the furnace annealing process,it is performed in a N₂O gas atmosphere at 800-1200° C. for about lessthan one hour. During this annealing process, nitrogen atoms (N) in thesemiconductor substrate are moved to the interface between the oxidefilm and the semiconductor substrate to thus increase the nitrogenconcentration of the interface portion, and nitrogen atoms (N) in theoxide film go outside to thus decrease the nitrogen concentration of thegate oxide film.

FIG. 5 is a graph illustrating the changes in the number of nitrogenatoms in the gate oxide film in both cases of including the annealingprocess and not including the annealing process after the formation ofthe gate oxide film. That is, FIG. 5 shows the results of the SIMS(secondary ion mass spectroscopy) analysis after the annealing process.In FIG. 5, white plots (“□”, “◯”, “Δ”, “∇”) show the number of oxygenatoms, while black plots (“▪”, “”, “▴”, “▾”) show the number ofnitrogen atoms. In particular, plots “□” and “▪” show the number ofoxygen atoms and the number of nitrogen atoms, respectively, when theannealing is not performed. The plots “◯” and “” show the number ofoxygen atoms and the number of nitrogen atoms, respectively, when theannealing is performed in a N₂ atmosphere at 1050° C. for 30 seconds.The plots “Δ” and “▴” show the number of oxygen atoms and the number ofnitrogen atoms, respectively, when the annealing is performed in a NOatmosphere. The plots “∇” and “▾” show the number of oxygen atoms andthe number of nitrogen atoms, respectively, when the annealing isperformed in a N₂O atmosphere.

FIG. 5 illustrates the changes in the number of nitrogen atoms accordingto a sputtering time. In addition, the graph of FIG. 5 corresponds tothe profile of nitrogen atoms in the oxide film and the semiconductorsubstrate. In FIG. 5, it is assumed that a region (“A” zone) in whichthe number of oxygen atoms is large shows the depth profile of thedistribution of concentration of oxygen atoms in the oxide film region,a region (“C” zone) in which the number of oxygen atoms is small showsthe depth profile of the distribution of concentration of oxygen atomsin the silicon substrate region, and the middle region (“B” zone) showsthe profile of oxygen atoms in the interface portion between the oxidefilm and the semiconductor substrate.

As illustrated in FIG. 5, as the result of performing annealing in a N₂Oatmosphere at 1050° C. for 30 seconds by the rapid thermal annealingmethod, it is shown that the number of nitrogen atoms in the oxide film(“A” zone) is decreased as compared to the case of not performingannealing. This is because the nitrogen atoms are discharged to theoutside during the annealing process. In addition, it is shown that thenumber of nitrogen atoms is slightly increased in the interface portion(“B” zone) between the oxide film and the silicon substrate. This isbecause nitrogen ions in the semiconductor substrate move toward theinterface portion during the annealing process.

Meanwhile, in case of performing annealing in a NO atmosphere, there isalmost no change in the profile of nitrogen atoms in the oxide film ascompared to prior to the annealing, and the number of nitrogen atoms inthe interface between the semiconductor substrate and the oxide film isincreased.

On the contrary, in case of performing annealing in a N₂ atmosphere,there is no change in the profile of nitrogen atoms in the oxide film.

Thus, it is most appropriate that the annealing is performed in a N₂Oatmosphere so as to improve the characteristics of the semiconductordevice.

Meanwhile, FIG. 6 is a graph illustrating the increase of the amount ofleakage current after forming a gate oxide film and performingannealing. As illustrated therein, in case of performing the annealingin a N₂O atmosphere, it can be known that the amount of leakage currentis decreased as compared to the case of not performing the annealing.

In addition, FIG. 7 is a graph illustrating the characteristics of theoxide film, which shows the amount of electric charge (Qbd) accumulatedin the oxide film to the breakdown of the oxide film. In FIG. 7, theplot “▪” shows the case of not performing the annealing, the plot “”shows the case of performing the annealing, and the plot “▴” shows thecase of not performing a nitrogen ion implantation.

As illustrated therein, it can be known that the Qbd after the annealingis larger than the Qbd prior to the annealing, after the nitrogen ionimplantation.

Therefore, when the annealing is performed in a N₂O or O₃ atmosphereafter the nitrogen ion implantation, the degree of degradation of theoxide film becomes lower, and the reliability of the oxide film becomehigher, as compared to the case of not performing the annealing.

According to the present invention, the concentration of nitrogen in thegate oxide film is decreased, and the concentration of nitrogen in theinterface between the gate oxide film and the semiconductor substrate isincreased, by additionally including an annealing process after theformation of the gate oxide film, in fabricating multiple gate oxidefilms using a nitrogen ion implantation. As the result, there is aneffect of decreasing the degradation of the gate oxide film and theamount of leakage current for thereby increasing the reliability of thegate oxide film. In addition, since the concentration of nitrogen in theinterface between the gate oxide film and the semiconductor substrate isincreased, in case of a P-MOS transistor, boron in a gate electrode isprevented from penetrating into the semiconductor substrate, thus makingthe change in threshold voltage of the transistor and improving thecharacteristics of the semiconductor device.

As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the meets and bounds of theclaims, or equivalences of such meets and bounds are therefore intendedto be embraced by the appended claims.

What is claimed is:
 1. A method for fabricating a gate oxide film of asemiconductor device, comprising the steps of: forming a screen oxidefilm on a top surface of a semiconductor substrate; forming an ionimplantation mask on parts of a top surface of the screen oxide film;implanting nitrogen ions into the semiconductor substrate using the ionimplantation mask; removing the ion implantation mask and the screenoxide film; forming an oxide film on the top surface of thesemiconductor substrate; and annealing the semiconductor substrate. 2.The method according to claim 1, wherein the method further comprises apre-annealing step after the ion implantation step.
 3. The methodaccording to claim 2, wherein the pre-annealing step is a process ofannealing at 500-900° C. by a furnace annealing method.
 4. The methodaccording to claim 2, wherein the pre-annealing step is a process ofannealing at 850-1200° C. by a rapid thermal annealing method.
 5. Themethod according to claim 1, wherein the oxide film formation step is amethod for thermal oxidation at a furnace of 700-950° C.
 6. The methodaccording to claim 1, wherein the oxide film formation step is a methodfor thermal oxidation at 850-1200° C. at a rapid thermal annealingmethod.
 7. The method according to claim 1, wherein the annealing stepis performed by means of a rapid thermal annealing method in a N₂Oatmosphere at a temperature of 900-1200° C. for about less than fiveminutes.
 8. The method according to claim 1, wherein the annealing stepis performed by means of a thermal annealing method in an O₃ atmosphereat a temperature of 400-1200° C. for about five minutes.
 9. The methodaccording to claim 1, wherein the annealing step is performed by meansof a furnace annealing method in a N₂O atmosphere at a temperature of850-1200° C. for about one hour.
 10. The method according to claim 3,wherein the annealing step is performed by means of a rapid thermalannealing method in a N₂O atmosphere at a temperature of 900-1200° C.for about less than five minutes.
 11. The method according to claim 3,wherein the annealing step is performed by means of a rapid thermalannealing method in an O₃ atmosphere at a temperature of 400-1200° C.for about five minutes.
 12. The method according to claim 4, wherein theannealing step is performed by means of a furnace annealing method in aN₂O atmosphere at a temperature of 850-1200° C. for about one hour. 13.The method according to claim 4, wherein the annealing step is performedby means of the rapid thermal annealing method in a N₂O atmosphere at atemperature of 900-1200° C. for about less than five minutes.
 14. Themethod according to claim 4, wherein the annealing step is performed bymeans of a rapid thermal annealing method in an O₃ atmosphere at atemperature of 400-1200° C. for about five minutes.
 15. The methodaccording to claim 4, wherein the annealing step is performed by meansof a furnace annealing method in a N₂O atmosphere at a temperature of850-1200° C. for about one hour.